The present invention relates to an ink jet recording apparatus for ejecting an ink drop from a nozzle orifice by causing pressure fluctuation in a pressure chamber to record an image, and relates to a method of driving the apparatus.
In an ink jet recording apparatus such as an ink jet printer or a plotter, an ink drop is ejected from a nozzle orifice of a recording head, thereby recording an image or a character on recording paper. More specifically, the recording head is moved in a main scanning direction and the recording paper is moved in a subscanning direction. The ink drop is ejected by operating a pressure generating element (for example, a piezoelectric vibrator) provided in association with the nozzle orifice and causing pressure fluctuation in a pressure chamber communicating with the nozzle orifice.
In this related recording apparatus, gradation recording is carried out with plural kinds of dots having various sizes in order to enhance an image quality. For this reason, there has been proposed a recording apparatus for changing the size of a dot according to the number of ejecting pulses to be supplied to a pressure generating element. In the recording apparatus, a drive signals in which a plurality of ejecting pulses having the same shape for electing an ink drop are arranged in series is generated, and at least one of the pulse signals is selectively supplied to the pressure generating element from the drive signal.
For example, a drive signal includes three ejecting pulses generated at constant intervals within a unit recording period. In a case where a large dot is to be formed, all the three ejecting pulses are supplied to the pressure generating element in a case where a medium dot is to be formed, two ejecting pulses are supplied to the pressure generating element. In a case where a small dot is to be formed, one ejecting pulse is supplied to the pressure generating element. Consequently, the recording is carried out in four gradations of xe2x80x9clarge dotxe2x80x9d, xe2x80x9cmedium dotxe2x80x9d, xe2x80x9csmall dotxe2x80x9d and xe2x80x9cnon-recordingxe2x80x9d.
In the related recording apparatus, thus, the pressure fluctuation is generated on ink in the pressure chamber to eject an ink drop. Accordingly, it is important that conditions for ejecting each ink drop are coincident with each other to record a high quality image consist of dots having identical sizes.
There will be considered a case where an ejecting pulse is generated in a cycle of 50 microseconds and three ejecting pulses are included in a unit recording period. In this case, when a large dot is to be recorded, all the three ejecting pulses in the recording period are selected. When a medium dot is to be recorded, two ejecting pulses, for example, a first ejecting pulse and a third ejecting pulse are selected. Moreover, when a small dot is to be recorded, one ejecting pulse, for example, a second ejecting pulse is selected.
In a case where the large dot is to be recorded successively, all the ejecting pulses generated in the cycle of 50 microseconds are selected and are supplied to the pressure generating element. Therefore, an interval at which the ejecting pulse is supplied becomes constant to be 50 microseconds. Similarly, when the small dot is to be recorded successively, a specific ejecting pulse is selected to be supplied to the pressure generating element. For this reason, the interval at which the ejecting pulse is supplied becomes constant to be 150 microseconds. Referring to the large dot and the small dot, accordingly, the amount of the ink drop can be equal also during the successive recording operation.
In a case where a medium dot is to be recorded successively, however, two of the three ejecting pulses generated at constant intervals in a unit recording period are selected. Therefore, the interval at which the ejecting pulse is supplied is not constant with a variation of 50 microseconds, 100 microseconds, 50 microseconds, 100 microseconds, . . . and vice versa.
Due to such a variation in the supply interval, the ink drop is ejected unstably. For example, flight deviation of ink drop is occurred or the amount of the ink drop is varied. It is supposed that such a situation occurs because the pressure fluctuation in the pressure chamber becomes unstable with variation in the supply interval. When the ink drop is ejected unstably, there is a drawback that an image has uneven gradations.
The invention has been made in consideration of such circumstances and has an object to provide an ink jet recording apparatus capable of preventing a drawback such as uneven gradations in a recorded image and to provide a method of driving the apparatus.
In order to achieve the above object, according to the present invention, there is provided an ink jet recording apparatus, comprising:
an ink jet recording head, including a nozzle orifice, a pressure chamber communicated with the nozzle orifice, and a pressure generating element which generates pressure fluctuation in ink stored in the pressure chamber to eject an ink drop from the nozzle orifice;
a drive signal generator, which generates a series of drive signals, each drive signal including at least three primary ejecting pulses generated at a first cycle in a unit recording period and at least one auxiliary ejecting pulse generated after a time period which is a half of the first cycle is elapsed since one of the primary ejecting pulse is generated, the primary ejecting pulses and the auxiliary ejecting pulse having an identical waveform; and
a pulse supplier, which selectively applies at least one of the primary ejecting pulses and the auxiliary pulse from the drive signal to the pressure generating element so that the number of ink drop ejected in the unit recording period is changed in accordance with a size of dot to be recorded, the pulse supplier selecting supplied pulse such that time intervals of the supplied pulses are made constant over adjacent unit recording periods when dots having the same size are successively recorded.
In this configuration, the state of meniscus of ink at a time when each ejecting pulse is started to be supplied cam be made equal so that the ejecting conditions of the ink drop is fixed. Thereby, variation in the ejection amount, the flight speed and the flight direction of the ink drop can be prevented. As a result it is possible to prevent a recorded image from being uneven, thereby enhancing the recorded image quality.
Preferably, three primary ejecting pulses and an auxiliary ejecting pulse are included in each drive signal. Here, the pulse supplier selects the three primary ejecting pulses from the drive signal when a dot having a first size is recorded. The pulse supplier selects one of the primary ejecting pulses and the auxiliary ejecting pulse from the drive signal when a dot having a second size which is smaller than the first size is recorded.
Here, it is preferable that the pulse supplier selects one of the primary ejecting pulses which is secondary generated when a dot having a third size which is smaller than the second size is recorded. And the primary ejecting pulse selected when the second size of dot is recorded is a primary ejecting pulse placed in opposite side of the secondary generated primary ejecting pulse with respect to the auxiliary ejecting pulse.
Further, It is preferable that a timing which is a half of the unit recording period comes while a primary ejecting pulse appeared secondary is generated.
Preferably, the drive signal includes a vibrating pulse which is selectively applied to the pressure generating element by the pulse supplier for vibrating meniscus of ink in the nozzle orifice such an extent that an ink drop is not ejected therefrom. Here, the vibrating pulse is generated between at least one pair of the adjacent primary ejecting pulses at which no auxiliary ejecting pulse is generated.
Preferably, the pressure generating element is a piezoelectric vibrator which varies a volume of the pressure chamber.
Alternatively, it is preferable that the pressure generating chamber is a heating element which varies a volume of an air bubble generated in ink stored in the pressure chamber with heat.
According to the present invention, there is also provided a method of driving an ink jet recording apparatus provided with an ink jet recording head, inducting a nozzle orifice, a pressure chamber communicated with the nozzle orifice, and a pressure generating element the method comprising the steps of:
generating a series of drive signals, each drive signal including at least three primary ejecting pulses generated at a first cycle in a unit recording period and at least one auxiliary ejecting pulse generated after a time period which is a half of the first cycle is elapsed since one of the primary ejecting pulse is generated, the primary ejecting pulses and the auxiliary ejecting pulse having an identical waveform;
selecting at least one of the primary ejecting pulses and the auxiliary pulse from the drive signal such that the number of ink drop ejected in the unit recording period is changed in accordance with a size of dot to be recorded, and such that time intervals of the supplied pulses are made constant over adjacent unit recording periods with respect to all recordable gradation levels; and
applying the selected ejecting pulse to the pressure generating element to generate pressure fluctuation in ink stored in the pressure chamber to eject an ink drop from the nozzle orifice.
Preferably, three primary ejecting pulses and an auxiliary ejecting pulse are included in each drive signal so that a first gradation level realized by a dot having a first size, a second gradation level realized by a dot having a second size which is smaller than the first size, and a third gradation level realized by a dot having a third size which is smaller than the second size are made recordable. Here, the three primary ejecting pulses are selected from the drive signal when the first gradation level is recorded. One of the primary ejecting pulses and the auxiliary ejecting pulse are selected from the drive signal when the second gradation level is recorded. One of the primary ejecting pulses is selected from the drive signal when the third gradation level is recorded.
Here, it is preferable that the primary ejecting pulse selected when the third gradation level is recorded is a primary ejecting pulses which is secondary generated. And it is preferable that the primary ejecting pulse selected when the second gradation level is recorded is a primary ejecting pulse placed in opposite side of the secondary generated primary ejecting pulse with respect to the auxiliary ejecting pulse.
Preferably, the pressure fluctuation is generated by varying a volume of the pressure chamber through use of a piezoelectric vibrator as the pressure generating element.
Alternatively, it is preferable that the pressure fluctuation is generated by varying a volume of an air bubble generated in ink stored in the pressure chamber with heat generated from a heating element provided as the pressure generating element.